2XMM Ultraluminous X-Ray Source Candidates in Nearby Galaxies

TL;DR

This study compiles a large, improved catalog of 650 ULX candidates in 238 nearby galaxies, analyzing their luminosity functions and distribution patterns to better understand their nature and relation to host galaxy properties.

Contribution

It provides a significantly expanded and refined catalog of ULX candidates, with background contamination estimates and population analysis, advancing the understanding of ULX luminosity functions and host galaxy correlations.

Findings

The luminosity function follows a simple power-law.

No statistical evidence for a high luminosity cutoff.

ULX occurrence per galaxy mass varies with galaxy type.

Abstract

Ultraluminous X-ray sources (ULXs) are some of the most enigmatic X-ray bright sources known to date. It is generally accepted that they cannot host black holes as large as those associated with active galaxies, but they appear to be significantly more luminous than their better understood Galactic X-ray binary (XRB) cousins, while displaying an intriguing combination of differences and similarities with them. Through studying large, representative samples of these sources we may hope to enhance our understanding of them. To this end, we derive a large catalogue of 650 X-ray detections of 470 ULX candidates, located in 238 nearby galaxies, by cross correlating the 2XMM Serendipitous Survey with the Third Reference Catalogue of Bright Galaxies. The presented dedicated catalogue offers a significant improvement over those previously published both in terms of number and the contribution of background contaminants, e.g. distant quasars, which we estimate to be at most 24 per cent, but more likely ~17 per cent. To undertake population studies, we define a `complete' sub-sample of sources compiled from observations of galaxies with sensitivity limits below 10^39 erg/s. The luminosity function of this sample is consistent with a simple power-law. Although we do not find any statistical requirement for a high luminosity cut-off, as has been reported previously, we are not able to rule out its presence. Also, we find that the number of ULXs per unit galaxy mass decreases with increasing galaxy mass for ULXs associated with spiral galaxies, in broad agreement with previous results, but is consistent with being constant with galaxy mass for sources associated with elliptical galaxies.